The theoretical relationship of drug pharmacokinetics and carbon dioxide breath test results has been investigated, using the well-stirred model of liver metabolism. The effects of hepatic extraction ratio, fraction metabolized by demethylation, and absorption rate of drug upon breath test results were simulated, using the MLAB package on the DEC 10 computer. It was demonstrated that a drug suitable for detecting changes in drug-metabolizing status via the carbon dioxide breath test should be at least 50% metabolized by demethylation, should have an hepatic extraction ratio of 0.2 to 0.5, and should be administered orally in a form that is rapidly absorbed.